Phoropters are apparatus used by optometrists to determine a patient's visual characteristics, so that proper eye diagnoses can be made and eyewear can be prescribed. In conventional phoropters, a patient looks through the phoropter, in which various test lenses are disposed, at a target eye chart, referred to as a “Snellen chart”, while an optometrist moves the test corrective lenses into the patient's field of view. In some applications, the target may be positioned at a predetermined distance from the patient. The patient is then asked to verbally compare the quality of the perceived image as afforded by one lens versus the prior lens presented. The optometrist takes note of either an improvement or a deterioration in the patient's vision through such lenses. Systematically, the test progresses towards the “best” test lens entirely based on the patient's responses. The lens parameters are then used as the basis for a prescription for eyewear.
Unfortunately, as recognized herein the patient can become fatigued during the process and/or misjudge the vision afforded by the various lenses. This can lead to the selection of a less than optimum prescription. Moreover, some patients, such as a very ill or a very young patient, might not be capable of articulating the quality of vision the various lenses afford the patient.
Objective methods of determining the patient's refraction errors have been proposed, but these other methods introduce further complications that are not present when using phoropters. In a retinoscopy method, for example, a streak of light is projected to a patient's retina, and the characteristics of the reflected light at the patient's corneal plane is analyzed to determine whether the patient is myopic, or hyperopic, and with or without astigmatism. However, the method does not provide sufficient accuracy for prescribing spectacle lenses. Consequently, its measurement results are typically used only as a starting point of a standard phoropter measurement.
Another objective measurement instrument for determining refractive errors is an autorefractor, which, owing to its speed of use, is more popular than retinoscopy. To use the autorefractor, a patient is asked to look inside an enclosed box that is part of the autorefractor. A target image is optically projected into patient's eye, and a series of lenses is automatically moved into position of the patient's line of sight to the target, to neutralize the patient's refractive errors (autorefraction). Unfortunately, the measurement outcome often differs from the patient's ideal prescription. Accordingly, like retinoscopy, autorefractor outcomes typically are used only as starting points for standard phoropter measurements.
Moreover, both retinoscopy and autorefraction fail to account for the accommodation effect of the patient, that is, for the propensity of a patient to alter his or her focus or sight to make the best of the vision test. An autorefractor measurement essentially is a snapshot of the patient's vision at a particular instant at which the autorefractor has identified a so-called neutralization point, and at this point if it happens that the patient focuses his vision for seeing an image at a distance other than what is intended, or if the patient is momentarily looking elsewhere other than the target, the output of the autorefractor is erroneous. Such deceptive focussing on the part of the patient can arise because the patient is conscious of the working distance inside the box, and when an image of an object presented to the patient which is modelled to be located at, e.g., twenty feet, the patient automatically focusses for an image at a much closer distance, knowing the actual size of the box. Examination results that include patient accommodation effects are inaccurate for prescribing spectacle lenses.
Another limitation of the autorefractor is that the examiner has no control over which lens is to be used in test. The result is that repeated measurements are likely to provide different results for the same eye from the same patient, which results in laborious and time consuming tests and retests when using the device to finalize a prescription. The present invention, having made the above-noted critical observations, provides the solutions disclosed herein.